Jones et al. Progress in Earth and Planetary Science (2020) 7:17 Progress in Earth and https://doi.org/10.1186/s40645-020-00327-1 Planetary Science RESEARCH ARTICLE Open Access Subducted oceanic crust as the origin of seismically slow lower-mantle structures TimothyD.Jones1*, Ross R. Maguire2,3, Peter E. van Keken1,JeroenRitsema4 and Paula Koelemeijer5 Abstract Mantle tomography reveals the existence of two large low-shear-velocity provinces (LLSVPs) at the base of the mantle. We examine here the hypothesis that they are piles of oceanic crust that have steadily accumulated and warmed over billions of years. We use existing global geodynamic models in which dense oceanic crust forms at divergent plate boundaries and subducts at convergent ones. The model suite covers the predicted density range for oceanic crust over lower mantle conditions. To meaningfully compare our geodynamic models to tomographic structures, we convert them into models of seismic wavespeed and explicitly account for the limited resolving power of tomography. Our results demonstrate that long-term recycling of dense oceanic crust naturally leads to the formation of thermochemical piles with seismic characteristics similar to the LLSVPs. The extent to which oceanic crust contributes to the LLSVPs depends upon its density in the lower mantle for which accurate data is lacking. We find that the LLSVPs are not composed solely of oceanic crust. Rather, they are basalt rich at their base (bottom 100–200 km) and grade into peridotite toward their sides and top with the strength of their seismic signature arising from the dominant role of temperature. We conclude that recycling of oceanic crust, if sufficiently dense, has a strong influence on the thermal and chemical evolution of Earth’s mantle. Keywords: Geodynamical modeling, Mantle structure, Seismic tomography Introduction of recycled oceanic crust (e.g., Christensen and Hofmann Tomographic models reveal the presence of two large 1994; Nakagawa et al. 2009, 2010). Markedly different anomalous structures in the lowermost mantle. These interpretationsofLLSVPspersistbecausethereisno large low-shear velocity provinces (LLSVPs) are sur- unique cause of seismic velocity anomalies in the man- rounded by high shear-wave velocity regions that are tle, but there are many plausible ones. For example, the associated with past subduction (Fig. 1; Tanaka et al. 2009; low-shear velocity anomaly (δ ln Vs ≈−2%) associated Ritsema et al. 2011; and and Cottaar and Lekic 2016). with LLSVPs may be equally well explained by an increase The origin of the LLSVPs and their connection to man- in temperature (Schuberth et al. 2009), a compositional tle dynamics and the long-term chemical evolution of the change (McNamara and Zhong 2005),thepresenceof planet remain unclear (see McNamara (2019) for a recent melt (Lee et al. 2010), or a combination thereof (Li and overview). Suggested origins include large-scale thermal McNamara 2013; Ballmer et al. 2016). The elevated ratio upwellings (e.g.,Schubert et al. 2004;Daviesetal.2015; between shear and compressional wavespeed anomalies and Koelemeijer et al. 2017), dense piles of primordial within LLSVPs (Robertson and Woodhouse 1995;Mas- and compositionally distinct mantle (e.g., Garnero and ters et al. 2000; Romanowicz 2001; Moulik and Ekström McNamara 2008; Burke et al. 2008), and accumulations 2016) is often regarded as evidence that they are thermo- chemical in origin, but it can also indicate the presence *Correspondence: [email protected] of post-perovskite without requiring a contribution from 1Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 large-scale chemical variations (Koelemeijer et al. 2018). Broad Branch Road NW, Washington, DC, 20015 USA Measurements of solid-Earth tides (Lau et al. 2017)and Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Jones et al. Progress in Earth and Planetary Science (2020) 7:17 Page 2 of 16 Fig. 1 Depth slices of S40RTS (Ritsema, 2011) showing variations in shear-wave velocity, δ ln Vs, with respect to the radial average. Both positive and negative velocity anomalies are highest toward the CMB. The color intensity is proportional to the velocity amplitude up to a maximum value of 2% normal mode splitting functions (e.g., Ishii and Tromp temperature and composition to seismic velocity. We will 1999;Trampertetal.2004; Moulik and Ekström 2016; then test how well such velocity structures are resolved and Koelemeijer et al. 2016) suggest that the LLSVPs in the shear-wave velocity model, S40RTS (Ritsema et al. have a higher than average density across the bottom 2011), and the combined shear- and compressional-wave two thirds of their depth range. However, constraining velocity model, SP12RTS (Koelemeijer et al. 2016). the density from low-frequency data depends on the As oceanic lithosphere subducts, three distinct compo- chosen set of normal modes, on how CMB topogra- nents are recycled back into the mantle: a thin basaltic phy is modeled, and on the theoretical simplifications crustandathickerbaseofharzburgiteontopofthemore made to model the data (e.g., Koelemeijer et al. 2017; primitive peridotite from which the crust was extracted. Akbarashrafi et al. 2018). Due to convective stirring of these components, the man- In this study, we will test the hypothesis that LLSVPs tle composition is an evolving mixture of the three end- are accumulations of dense recycled oceanic crust. We member compositions. To simplify the description below, are motivated in particular by geodynamical studies that we will refer to end-member basaltic, harzburgitic, and show that the long-term recycling of oceanic crust can sig- peridotitic compositions as “basalt”, “harzburgite”, and nificantly contribute to the formation of warm but dense “peridotite”, respectively, and, where relevant, will use pools of compositionally distinct material at the base of subscripts B, H,andP to identify them. the mantle (Fig. 2; Christensen and Hofmann 1994;Bran- Mineral physics experiments indicate that the basaltic denburg and van Keken 2007; Brandenburg et al. 2008; component is denser than ambient mantle through most and Nakagawa and Tackley 2008). We will focus on how of Earth’s mantle with a relative density contrast δ ln ρ well thermochemical model simulations predict the seis- of up to 2–7%, except near the surface and in the mological characteristics of LLSVPs by projecting the uppermost lower mantle (Ringwood and Irifune 1988; Jones et al. Progress in Earth and Planetary Science (2020) 7:17 Page 3 of 16 Fig. 2 Plot of temperature and composition fields for Brandenburg et al. (2008) models for δ ln ρ of 0% (a, b) and 3.8% (c, d), where we use the non-linear radial projection of the models onto Earth coordinates. In both cases, recently subducted oceanic crust meets the CMB and the low viscosity upper mantle has been homogenized by convective stirring relative to the lower mantle. When δ ln ρ = 0%, oceanic crust is somewhat evenly distributed throughout the lower mantle. In contrast, when δ ln ρ = 3.8%, oceanic crust accumulates and the CMB forms sharp piles Ringwood 1990;Hiroseetal.1999;AokiandTaka- at the CMB where it is swept into piles and mixes with the hashi 2004;Hiroseetal.2005; Ricolleau et al. 2010; ambient mantle. They concluded that this process could Tsuchiya 2011). Under these conditions, numerical exper- not only account for important features of Earth’s chem- iments predict that the oceanic crust will accumulate ical evolution but may also explain seismic heterogeneity at the core-mantle-boundary (CMB) (Christensen and in the D" layer. Hofmann 1994;Davies2002; Xie and Tackley 2004;Bran- Due to computational limitations at the time, CH94 denburg and van Keken 2007).Thepresenceofrecycled performed dynamical computations at a convective vigor oceanic crust as a major component of mantle plumes that is significantly below that of present day Earth and (Hofmann and White 1982) is empirical evidence that used a scaling relationship to extrapolate their findings at least some portion of subducted material reaches the to models with present-day plate velocities. Brandenburg CMB. and van Keken (2007) explicitly showed that this extrap- The models presented in this study follow the approach olation is an accurate approximation. Brandenburg et al. of Christensen and Hofmann (1994) (herein CH94) who (2008) (herein BB08) further extended the study of CH94 assumed that the basalt remains denser throughout the by exploring the effects of crustal recycling in models mantle and retains a constant density contrast with the with force-balanced plates and with Earth-like convective ambient mantle. CH94 found that at sufficiently large vigor. Their results confirmed the findings of CH94. They density excess, the subducted oceanic crust accumulates also showed that the formation and recycling of oceanic Jones et al. Progress in Earth and Planetary Science (2020) 7:17 Page 4 of 16 crust could contribute to the HIMU-DMM-EMI diver- and that concentrate by a factor of 8 when the oceanic sity of Ocean Island Basalt (OIB) and Mid-Oceanic Ridge crust is formed such that a ∼10-km-thick basaltic crust is Basalt (MORB) in multiple isotope systems.